The state of the art shows various suggestions for equipping electrochemical measuring cells such that controlled diffusion conditions are preset for the detection of a defined gas component in a gas mixture, i.e., e.g., of CO in its mixture with H.sub.2 S, on the one hand, and that the retention of "interfering" gas components, i.e., e.g., H.sub.2 S, is ensured, on the other hand.
On the one hand, so-called diffusion diaphragms, which are described in, e.g., EP 0016423 A1 and are to contribute to the optimization of the electrochemical measurement conditions, have been known for limiting the access of a gas component to be detected to the measuring cell. On the other hand, filters are inserted between the gas mixture and the measuring cell proper in order to retain, e.g., moisture, dust or especially also gas components which interfere with the measurement from the mixture and which would otherwise distort the measured signal of the measuring cell. A gas-measuring apparatus with a diffusion barrier with a capillary is disclosed in DE 27 09 903 C2.
The provision of a filter layer in the gas inlet for separating moisture for a calorimetric gas dosimeter is described in DE 34 45 638 C2.
The commercially available filters are either rigidly connected to the measuring cell and are not replaceable, or they cause interference signals due to leakage flows through the "bypass" between the filter housing and the measuring cell proper. These leakage flows are due to the fact that the filters are not fitted into the housing of the measuring cell in an absolutely gas-tight manner, and it is therefore hardly possible to prevent interfering gas components from flowing directly through this air gap because of the existing potential gradient and from reaching the measuring cell without flowing through the filter material and from causing an interference signal there.